Pr3+ ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses for visible red luminescent devices

Pr3+ ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses for visible red luminescent devices

Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: w...

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Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol

Pr3+ ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses for visible red luminescent devices ⁎

Ch.B Annapurna Devia, Sk. Mahamudaa, K. Swapnaa, M. Venkateswarlua, A. Srinivasa Raoa,b, , G. Vijaya Prakashc a

Department of Physics, Koneru Lakshmaiah Education Foundation, Green Fields, Vaddeswaram, 522 502 Guntur (Dt), Andhra Pradesh, India Department of Applied Physics, Delhi Technological University, Bawana Road, New Delhi 110 042, India c Department of Physics, Indian Institute of Technology-Delhi, Hauz Khas, New Delhi 110 016, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Glasses Praseodymium J-O parameters Optical properties Photoluminescence Radiative properties

The present work illustrates the optical absorption and fluorescence properties of Pr3+ ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses prepared by using melt quenching technique. The prepared glasses were characterized by using absorption, excitation and photoluminescence (PL) spectral measurements. The energies of the absorption spectral features called oscillator strengths were calculated using area method and in turn used to evaluate the Judd-Ofelt (J-O) intensity parameters (Ω2, Ω4, Ω6). Such J-O parameters are used to estimate various radiative parameters such as radiative transition probabilities (AR), branching ratios (βR), and radiative lifetimes (τR) for the prominent fluorescent levels of Pr3+ ions in these glasses. The PL spectra of the as prepared glasses show three prominent peaks at wavelengths 488, 646 and 670 nm related to the transitions 3P0 → 3H4, 3P0 → 3F2 and 1D2 → 3H5 respectively for which emission cross-sections and branching ratios were evaluated. The variations in the spectral parameters with the variation of glass matrix composition have been examined in detailed. The decay profiles of the prepared glasses have been recorded to evaluate quantum efficiency of present series of glasses. From the measured branching ratios, emission cross-sections and quantum efficiency, it was concluded that the tungsten tellurite glasses added with potassium fluoride and doped with 1 mol% of Pr3+ions (TeWK glass) are quite suitable to produce visible red emission at 670 nm suitable to fabricate red luminescent devices.

1. Introduction

quantum efficiency [11,12]. Tellurite glasses doped with RE ions have dragged so much attention for spectroscopic investigations, due to their potential applications in diversified areas like biochemical studies, telecommunications and optical sensors etc. [13–16]. Tellurite host matrices have several interesting features like low phonon energy (< 800 cm−1), high refractive index, large dielectric constant, broad transmission window (~0.4–5 μm), large RE ion solubility and higher stability among various other oxides hosts [17–26]. The above characteristics prompted us to choose tellurites as host matrix for the present investigation. On the other hand tellurite based glasses are conditional glass formers; i.e., independently they can't form a glass without the support of intermediates/modifiers such as WO3, Ti2O, Bi2O3, Li, Na and K [21]. Addition of tungsten oxide to tellurite's makes them non-hygroscopic and transparent in the visible and NIR regions. One of the special features of tungsten ions is that they show strong influence on luminescence properties of RE ions in the tellurite glasses because of their various valency states (W6+, W5+ and W4+). The

Recently Rare Earth (RE) ions doped oxide glasses are playing vital role in diversified fields such as photonics, optoelectronic devices, wavelength conversion devices, optical fiber amplifiers, fiber lasers and compact microchip lasers [1–4]. Glassy materials have got so much of attention when compared with crystalline materials for the aforementioned applications because of their low cost and simple manufacturing procedure. Over and above the bulk samples of glassy materials can be prepared easily when compared with single crystalline materials [5]. Many researchers have investigated the lasing potentialities of RE ions doped fluoride, borate, fluoroborate and tellurite glasses to understand the effect of the host material and doped RE ion concentration [6–10]. Among the various types of oxide glasses, tellurite based oxide glasses have gained so much of attention in recent years because of their unique properties such as low phonon energy and low melting point (8000 ± C); which are in fact the key points to get relatively higher



Corresponding author at: Department of Applied Physics, Delhi Technological University, Bawana Road, New Delhi 110 042, India. E-mail address: [email protected] (A. Srinivasa Rao).

https://doi.org/10.1016/j.jnoncrysol.2018.03.034 Received 23 November 2017; Received in revised form 24 January 2018; Accepted 12 March 2018 0022-3093/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Annapurna Devi, C.B., Journal of Non-Crystalline Solids (2018), https://doi.org/10.1016/j.jnoncrysol.2018.03.034

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TeWLi: 59TeO2 - 20WO3 - 20LiF - 1Pr6O11 TeWNa: 59TeO2 - 20WO3 - 20NaF - 1 Pr6O11 TeWK: 59TeO2 - 20WO3 - 20 KF - 1 Pr6O11 TeWLiNa: 59 TeO2 - 20WO3 - 10LiF - 10NaF - 1 Pr6O11. TeWLiK: 59TeO2 - 20WO3 - 10LiF - 10KF - 1 Pr6O11. TeWNaK: 59TeO2 - 20WO3 - 10NaF - 10KF - 1 Pr6O11. The analar grade chemicals of TeO2, WO3, LiF, NaF, KF & Pr6O11 were used as raw materials to prepare the glass materials. Approximately 10 g of each batch composition was weighed in an electrical balance and crushed in an agate mortar for 2 h to obtain uniform mixture. Then the mixture was collected in a silica crucible and it is sintered at 750 °C for ½ h and kept in a temperature controlled furnace and swirled inside the furnace to obtain homogeneous melt. The bubble free melt was then quenched in between a couple of preheated brass plates to get the glass samples with a uniform thickness. Later, these glass samples were annealed at 400 °C for 4 h to avoid the internal stresses generated by rapid cooling and to improve structural stability. The prepared samples are polished with emery paper before using them for absorption and luminescence spectral characterizations. For the prepared samples, densities are measured using standard Archimede's principle, using water as buoyant liquid. Refractive indices of the prepared glass samples were measured using the Brewster's angle method (He-Ne laser at λ = 650 nm) with an accuracy ± 0.01. The optical absorption spectra of the as prepared glasses have been recorded by using a double beam JASCO V-670 UV–vis-NIR spectrophotometer at room temperature with a spectral resolution of 0.1 nm. This is to be noted that the host glass matrix optical band gap is typically about 3 eV (i.e., absorption edge at 400 nm). While calculating the oscillator strengths (from areas under the absorption curve) and corresponding Judd-Ofelt parameters, appropriate background corrections have been made. The PL spectra of these glasses were recorded using RF-5301 PC Spectrofluorophotometer at room temperature. The decay profiles for the present series of glasses have been recorded using Edinburgh FLSP900 with a spectral resolution of 0.1 nm.

thermo-reversible disproportionate reaction of these ions is represented as W5+ + W6+ ↔ W4+ + W6+ [27–29]. Each valency state of tungsten ion has a unique importance depending upon the formation of bonds with tellurium oxide. Tungsten ions with 5+ valence state (W5+) form the complexes of W5+O3 −. Due to these complexes, structural changes happen in tellurite network because TeO4 structural units changes to TeO3+1 structural units [30,31]. The 6+ valence state of tungsten ions entered into the network of glasses as WO4 and WO6. Due to these units, new networks/linkages formed between Te ions and tungsten ions such as Te-O-W with TeO4 and TeO3 structural units [30,31]. In addition to the above, addition of WO3 to a tellurite glass enhances the chemical stability and devitrification resistance. This loosens the glass network and increases the solubility of RE ions in the host glass [32–34]. The present series of tungsten tellurite glasses have been prepared in combination with different alkali fluorides (RF; where R stands for Li, Na and K metals) which act as network modifiers (NWM) and increase the stability of the glasses [35]. Moreover alkali fluorides can avert the moisture in glasses more effectively than alkali oxides and iodides [36]. The fluoride ions in alkali fluorides act as co-activators and replace the activators in the lattice. Due to the addition of these alkali fluorides, the mixed alkali effect (MAE) raised which can change various physical properties such as dielectric loss, ionic conductivity and alkali diffusion coefficient due to cation movements [37–40]. In addition to the above, fluorides addition to the glasses can lower the phonon energy (300600 cm−1) which in turn reduces non-radiative decay losses due to multiphonon relaxations. If non-radiative losses are reduced, the quantum efficiency of RE ion doped glass will increase. Due to these low non-radiative decay rates near maximum infrared (IR) cut-off edge will occur, which can lowers the non-linear refractive index. Hence transmission ability enhances from UV to IR. Addition of alkali fluorides to these glasses decreases OH absorption because fluoride ions reacting with OH group will form HF [41–43]. Fluoride content added to these glasses can also decrease the phonon energy and enhance the emission cross-section and quantum efficiency. This has been reported by many researchers in Pr3+ ions doped host glasses [44,45]. Among different RE ions, Pr3+ ion is one of the optical activator which gives luminescence in blue, green, red and IR regions with numerous applications such as optical devices, up-conversion lasers and fiber amplifiers [46–53]. The Pr3+ ions have a number of metastable states which are able to provide the efficient luminescence from visible to infrared region. Recent studies have been focused on 1.3 μm (1G4→3H4) emission from Pr3+ions doped glasses which are very useful in telecommunication window [10,11,69]. Pr3+ ions also acts as an active luminescent centre, to give red luminescence in 600-720 nm wavelength region [11–25], which locates in the maximum absorption region of the Photo sensitizing currently used in photodynamic therapy or clinical trials [54,55]. Favourable red luminescence from Pr3+ ions doped glass fibers with sufficient intensity and suitable directivity are quite suitable for PDT treatment [54,55]. On the other hand, the studies on 3P0 → 3H6 (orange-red emission) are not as much focused even though this luminescence is very useful as signal light source in astronomical telescope, in medical therapies (ophthalmology for the treatment of retinal disorders, removal of tissues, tumours and in the treatment of acne and in skin restoration). Various scientific patronages offered by the chemical constituents such as TeO2, WO3 and alkali fluorides have prompted us to take up a systematic investigation on Pr3+ ions doped different compositions of single and mixed alkali fluoro tungsten tellurite glasses for orange-red luminescent device applications.

3. Results and discussions 3.1. Analysis of optical absorption spectra-measurement of oscillator strengths& J-O parameters In order to understand the radiative properties possessed by Pr3+ ions doped in the as prepared glasses, absorption spectra have been recorded from 400 to 2200 nm. Fig. 1 shows the absorption spectra of Pr3+ ions doped alkali and mixed alkali fluoro tungsten tellurite 3.0 3

3

P2

H4

TeWLi TeWNa TeWK TeWLiNa TeWLiK TeWNaK

2.5

Absorbance (a.u)

3

P1 3P

0

2.0

3

F3

3

F2

3

1.5

F4

1

D2 1

1.0

G4

0.5

2. Experimental

0.0 400

2.1. Synthesis and characterization of glasses

450

500

550

600

1000 1200 1400 1600 1800 2000 2200

Wavelength (nm) Fig. 1. Absorption spectra of Pr3+ ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses.

In the present work, the glass samples have been prepared by conventional melt quenching method with the following compositions. 2

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glasses. As shown in Fig. 1, the absorption spectra show eight absorption peaks related to the transitions 3H4 → 3P2, 3P1, 3P0, 1D2, 1G4, 3F4, 3 F3 and 3F2 belonging to 4f2 configuration at 447 nm, 472 nm, 485 nm, 599 nm, 1019 nm, 1451 nm, 1539 nm and 1944 nm respectively. All these transitions are assigned according to Carnall et al. [56] and assumed to be electric dipole transitions. The assignment of these absorption bands are in good agreement with the literature [21,56]. Among all the transitions, 3H4 → 3P2 (447 nm) transition is known as hypersensitive transition. This transition strongly depends upon the local environment around the RE ions. This hypersensitive transition is represented by the selection rule ΔS = 0, |ΔL| ≤ 2 and |ΔJ| ≤ 2 [57]. By using the absorption spectral data, the nature of bonding in the prepared glasses can be estimated in terms of nephelauxetic ratio (β ) and bonding parameter (δ). Depending upon the ligands, the value of bonding parameter is positive or negative which indicates covalent or ionic nature between the ligands. For the present series of glasses, the nephelauxetic ratio and bonding parameters are TeWNa – 0.9933 & 0.6671, TeWLi – 0.9931 & 0.6891, TeWK – 0.9924 & 0.7649, TeWLiK – 0.9956 & 0.4419, TeWNaK – 0.9942 & 0.5819 and TeWLiNa – 0.9938 & 0.6141. From the obtained bonding parameters values it is concluded that TeWK glass is more covalent when compared with other single alkali and mixed alkali glasses. These values are also comparable to the values reported to the other alkali and mixed alkali glasses like LiCdBS (1.2146), NaCdBS (1.0918), KCdBS (0.8573) [58]. The electric dipole oscillator strengths for all the peaks in the optical absorption spectra (fexp) have been evaluated using the expression given in the literature [21].Using these oscillator strengths and J-O theory, the calculated oscillator strengths have been evaluated using the equations available in the literature [21] with and without including the hypersensitive transition. As the deviations observed in calculated oscillator strengths with and without including oscillator strengths are very less, in Table 1 we gave only oscillator strengths with the inclusion of hypersensitive transition along with the experimental oscillator strengths and rms deviation values. From Table 1 it is observed that among all the glasses (single and mixed alkali) the TeWK glass possesses the maximum values of oscillator strengths and the magnitude of hypersensitive transition is also high for this glass. The required conditions for good lasing materials are higher magnitudes of branching ratios and stimulated emission cross-sections of particular luminescent level. Evaluation of these crucial parameters is possible using J-O theory. This theory was explained based on the following four assumptions [60].

Table 2 J-O Intensity Parameters (Ωλ = 10−20 cm2; λ = 2,4,6) of Pr3+ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses. Glass System

Ω2

Ω4

Ω6

Trend

TeWLi TeWNa TeWK TeWLiNa TeWLiK TeWNaK Glass A Oxy-fluoride Mixed halide Phosphate MgTP LBTAF ZBP3 Tellurite

8.37 7.20 12.75 10.08 8.56 9.39 4.78 0.13 2.70 4.19 2.69 2.42 2.89 3.81

7.43 6.72 11.18 10.59 9.55 9.64 6.15 4.09 4.40 4.29 11.8 2.54 2.04 5.81

2.07 2.04 3.37 3.26 2.80 2.81 2.21 6.33 5.40 6.40 8.39 4.23 1.99 4.10

Ω2 Ω2 Ω2 Ω4 Ω4 Ω4 Ω4 Ω6 Ω6 Ω6 Ω4 Ω6 Ω2 Ω4

> > > > > > > > > > > > > >

Reference Ω4 Ω4 Ω4 Ω2 Ω2 Ω2 Ω2 Ω4 Ω4 Ω4 Ω6 Ω4 Ω6 Ω6

> > > > > > > > > > > > > >

Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω6 Ω2 Ω2 Ω2 Ω2 Ω2 Ω4 Ω2

Present Present Present Present Present Present [21] [59] [61] [62] [63] [64] [65] [66]

work work work work work work

3. All stark levels are equally populated. 4. The material is optically isotropic For most of the RE ions, the first two assumptions are satisfied moderately which need to be simplified greatly. In Pr3+ ions, the 4f N 5d1 band is very low and very close to 3P2, 3P1, 1I6, and 3P0 levels, so that they perturb noticeably for the J-O calculation. The J-O parameters (Ωλ, λ = 2, 4 and 6) have been measured using least square fitting method by considering the hypersensitive transition 3H4 → 3P2 and are given in Table 2 along with the other reported values [21,59,61–66]. From Table 2, it is observed that the J-O intensity parameters are following two different trends for single alkali and mixed alkali tungsten tellurite glasses. Single alkali tellurite glasses follow the trend Ω2 > Ω4 > Ω6 whereas mixed alkali glasses follows a different trend Ω4 > Ω2 > Ω6. The J-O parameter Ω2 is a representation of covalency of metal ligand bond and other two parameters Ω4 and Ω6 represents the viscosity and rigidity of the host glass matrix. Among all the glasses (single and mixed alkali) the TeWK glass possesses higher values for the J-O parameters. Among the alkali ions, K+ has less ionization energy when compared with Li+ and Na+ ions. This helps potassium ions to act as a relatively good network modifier when compared with lithium and sodium ions. Consequently, the presence of potassium in a glass can produce more number of non-bridging oxygen's when compared with the lithium and sodium ions. This makes the glasses containing potassium (a good network modifier) to give relatively higher values for JO parameters when compared with other alkali ions [67–69]. These values are found to be high when compared with other reported glasses such as oxy-fluoride, mixed halide, phosphate, MgTP, LBTAF, ZBP3, Glass A and tellurite [21,59,61–66] glasses. The larger values of Ω2 parameter of TeWK glass suggests that, the bonding

1. An average energy is assumed for excited configurations above the 4fN 2. The energy difference between two states J and J' is small when compared to the difference between the energy of the 4fN configuration and the energy of the 4fN5d1configuration.

Table 1 Experimental (fexp) and calculated (fcal) oscillator strengths (×10−6), r.m.s deviations (δrms), refractive indices and density (g/cm3) of Pr3+ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses. Transition from 3H4 →

3

P2 P1 3 P0 1 D2 1 G4 3 F4 3 F3 3 F2 δrms(×10−6) n d 3

TeWLi

TeWNa

TeWK

TeWLiNa

TeWLiK

TeWNaK

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

13.5 5.64 5.49 8.78 1.98 1.21 11.9 13.5 ± 5.77 2.463 5.518

0.41 9.49 9.35 1.52 0.42 3.74 10.53 13.59

10.0 4.18 4.82 8.55 1.32 0.96 10.8 11.5 ± 4.77 2.423 5.546

0.36 8.31 8.18 1.38 0.38 3.45 9.40 11.6

22.1 9.75 8.16 13.0 3.31 2.52 18.0 20.4 ± 4.77 2.423 5.546

0.61 14.1 13.9 2.36 0.65 5.90 16.1 20.5

18.55 8.50 7.45 11.74 2.48 2.12 16.59 17.19 ± 7.78 2.426 5.532

0.57 13.15 12.95 2.17 0.59 5.45 14.72 17.31

15.73 6.75 6.28 11.52 11.52 2.54 1.27 14.97 ± 7.04 2.423 5.502

0.51 11.80 11.65 1.91 1.91 0.52 4.71 13.00

16.26 7.45 6.29 11.60 2.21 1.54 15.38 15.92 ± 7.15 2.436 5.516

0.52 11.99 11.83 1.96 0.54 4.87 13.49 17.04

3

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3

P1

75

0 420

430

440

450

460

470

480

490

500

510

1

17500

P0 -1

P2

20000

Energy (cm )

Intensity (a.u)

3 3

D2

3

P2 Photoluminescence 3P 3 1 P0

22500

D2

15000 12500 10000

1

7500

3

G4 F

4 3 3 3 2 3

F F H6

5000

520

Wavelength (nm) 3+

Fig. 2. Excitation spectra of Pr sten tellurite glasses.

2500

ions doped single alkali and mixed alkali fluoro tung-

3

H5

3

H4

0 3+

between Pr ions and the ligands is highly covalent in nature and the RE ion sites are having lower asymmetry [59] in these base glasses than the other reported glasses in literature [21,59–66]. This result is also in consistent with the result obtained from nephelauxetic ratio and bonding parameter.

3+

Fig. 4. Energy level diagram of Pr tungsten tellurite glasses.

Fig.2 represents the excitation spectra of TeWK glass at an emission wavelength 634 nm in the range 420–520 nm. It has four excitation bands corresponding to the transitions 3H4 → 3P2, 3H4 → 3P1, 3H4 → 3P0 & 3H4 → 1D2 at wavelengths 429 nm, 446 nm, 472 nm and 485 nm respectively. Among all the excitation bands, the band at 429 nm corresponding to the transition 3H4 → 3P2 is more intense and is used as an excitation wavelength to record emission spectra. Fig.3 shows the PL spectra recorded for all the glasses under 429 nm excitation in the wavelength range 475–750 nm. The PL spectra of all the glasses show six luminescence bands at wavelengths 488, 529, 616, 646, 670 and 730 nm corresponding to the transitions 3P0 → 3H4, 3P0 → 3H5, 3P0 → λexc= 429 nm

140 3

3

Intensity (a.u.)

P

0

D 2

TeWLi TeWNa TeWK TeWLiNa TeWLiK TeWNaK

160

120

3

1

180

H 4

3

H 5

3 P

F 2

0

100 80 60 3 3

40

3 P

0

3 P

H 5

0

H 6 3

3 P

20

0

ions doped single alkali and mixed alkali fluoro

3 H6, 3P0 → 3F2, 1D2 → 3H5 and 3P0 → 3F4 respectively. Among all these transitions, two transitions are highly intense in bluish-green and bright red regions at 488 nm and 670 nm corresponding to the transitions 3 P0 → 3H4 and 1D2 → 3H5 for TeWK glass. These two transitions are originating from 3P0 and 1D2 and they are well resolved. The bluishgreen emission from Pr3+ ions doped glasses can be explained on the basis of its energy level structure. The energy gap between 3P0 and 1D2 is 4042 cm−1. In the present series of glasses, red luminescence dominates bluish-green luminescence because of non-radiative relaxation that took place from 3P0 to 1D2. In some cases bluish-green luminescence dominates the red luminescence because of increased radiative transition from 3P0 to ground state instead of non-radiative relaxation of 3P0 to 1D2 state. The PL mechanism involved in the present work can be understood from Fig.4. In order to understand the PL performance of Pr3+ ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses, the various radiative parameters have to be evaluated. For the present series of glasses radiative transition probability (AR), radiative and experimental branching ratios (βR & βexp) were measured for the observed emission transitions by coupling the J-O parameters with PL data and are given in Table 3 along with the emission peak wavelength (λP), effective band widths (ΔλP), stimulated emission cross-section (σse), gain band width (σse × ΔλP) and optical gain parameters (σse × τR). The necessary mathematical expressions required to evaluate the above mentioned radiative parameters were collected from literature [56]. The most essential condition for a particular transition to act as an efficient laser transition is having higher values for branching ratio and stimulated emission cross-sections [70]. From the Table 3, it is observed that, the transitions 3P0 → 3H4 and 1D2 → 3H5 possess higher values of branching ratio, stimulated emission crosssections for TeWK glass. Hence, in the present series of glasses (Single and Mixed Alkali), TeWK glass is more suitable as lasing material to give bright red luminescence at 670 nm.

3.2. Analysis of photoluminescence spectra and evaluation of radiative properties

200

670 nm

1

150

25000

TeWLi TeWNa TeWK TeWLiNa TeWLiK TeWNaK

488 nm 529 nm 616 nm 640 nm 730 nm

λem= 634 nm

H4

Excitation

3

Absorption

225

F 4

3.3. PL decay spectral analysis

0 475

500

525

550

575

600

625

650

675

700

725

To identify the better luminescent material, another important parameter is quantum efficiency which can be evaluated from the radiative and experimental lifetimes. Radiative lifetime can be measured from the J-O analysis and the experimental lifetime can be measured from decay spectra. Hence the PL decay spectra for all the glasses have

750

Wavelength (nm) Fig. 3. Photoluminescence spectra of Pr3+ ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses.

4

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Table 3 Emission peak wavelength (λP) (nm), effective band widths (ΔλP) (nm), transition probability (AR), Branching ratios (βR&βexp) stimulated emission cross-sections (σse) (cm2), gain band width (σse × ΔλP) (cm3) and optical gain (σse × τR) (cm2 s) for the emission transitions of Pr3+ ions in single alkali and mixed alkali fluoro tungsten tellurite glasses. Spectral parameters 3 P0 → 3H4, λP = 488 nm ΔλP AR βR βexp σse(×10−18) σse × ΔλP (×10−25) σse × τR(×10−20) 3 P0 → 3F2, λP = 646 nm ΔλP AR βR βexp σse(×10−18) σse × ΔλP (×10−25) σse × τR(×10−20) 1 D2 → 3H5, λP = 670 nm ΔλP AR βR βexp σse(×10−18) σse × ΔλP (×10−25) σse × τR(×10−20)

TeWLi

TeWNa

TeWK

TeWLiNa

TeWLiK

TeWNaK

16.4 146,240 0.476 0.286 11.11 1.81 3.31

17.9 124,021 0.482 0.291 8.88 1.59 2.66

8.82 217,371 0.472 0.323 30.8 2.72 6.16

15.2 196,810 0.493 0.361 16.56 2.52 3.30

15.7 175,850 0.516 0.415 25.50 2.25 5.10

16.2 182,101 0.500 0.369 14.28 2.31 2.84

9.65 118,536 0.385 0.216 46.8 4.51 14.0

9.98 96,314 0.374 0.238 38.9 3.79 11.4

9.75 179,146 0.389 0.159 93.6 6.88 14.1

9.98 143,841 0.360 0.168 53.4 5.65 11.3

9.35 113,704 0.333 0.177 70.5 4.47 9.57

9.35 128,319 0.352 0.214 47.8 4.99 10.7

5.96 84,475 0.301 0.145 62.4 3.72 18.7

6.08 69,343 0.295 0.125 51.9 3.16 15.5

7.35 127,175 0.302 0.231 76.9 5.65 15.3

6.84 105,130 0.288 0.182 69.8 4.77 13.9

7.45 85,852 0.275 0.125 53.2 3.91 10.6

5.15 94,671 0.284 0.109 82.8 4.26 16.5

been recorded for the luminescent level 1D2 → 3H5 of Pr3+ ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses at an excitation wavelength of 429 nm. All the decay profiles are fitted in single exponential and Fig.5 shows the decay profile of TeWK glass. The experimental lifetimes (τexp) for the luminescent level 1D2 → 3H5 for all the glasses under investigation are tabulated in Table 4 along with the radiative lifetimes (τR). From the Table 4, it is observed that the experimental lifetimes (τexp) are smaller than the radiative lifetimes (τR). The experimental lifetimes are also compared with other reported values [71,72]. The variations between experimental and radiative lifetimes may be due to the non-radiative relaxation rates (WNR) of excited state which includes multi-phonon relaxation (MPR), energy transfer through cross-relaxation (CR) and several other non-radiative processes. The non-radiative relaxation rates (WNR) of present series of glasses also given in Table 4. The ratio of number of photons emitted to the number of photons absorbed is known as luminescence quantum efficiency (η). For RE doped materials it is equal to the ratio of the experimental lifetime to the predicted radiative lifetime for respective levels. The quantum efficiency values measured for all the glasses under investigation are given in Table 4. From the evaluated quantum efficiency values, it has been observed that, TeWK glass possesses maximum value of quantum efficiency. Hence, in the present series of Pr3+ ions doped single alkali and mixed alkali glasses, TeWK glass is quite suitable for the emission of laser in bright red region (670 nm).

1 TeWK Glass

Logarithmic Intensity (a.u)

Fitted Curve

0.1

0

5000

10000

15000

20000

25000

30000

Time ( μs)

Fig. 5. The photoluminescence decay spectrum of Pr3+ ions doped TeWK glass for 5D0 → 3 H5 (670 nm) emission transition.

Table 4 The Experimental (τexp) (μs), radiative (τR) (μs) lifetimes, quantum efficiencies (η) and non-radiative decay rates (WNR) (×106 s−1) for1D2 → 3H5 transition of Pr3+ions doped single alkali and mixed alkali fluoro tungsten tellurite glasses under 429 nm excitation. Glass

τexp

τR

η

WNR

Reference

TeWLi TeWNa TeWK TeWLiNa TeWLiK TeWNaK Li Na K LiK NaK 2.5Pr NaF

0.491 0.705 1.450 0.896 0.976 1.014 0.120 0.231 0.305 0.160 0.251 0.206

3 3 2 2 2 2 – – – – – –

16.3 23.5 72.5 44.8 48.8 50.7 – – – – – –

1.703 1.085 0.269 0.616 0.524 0.486 – – – – – –

Present Present Present Present Present Present [71]

4. Conclusions Good optical quality single alkali and mixed alkali fluoro tungsten tellurite glasses doped with Pr3+ions have been fabricated using conventional melt quenching technique. From the absorption spectra the JO intensity parameters have been evaluated. In the present series of glasses, the single alkali based glasses follows the trend Ω2 > Ω4 > Ω6 whereas mixed alkali based glasses follows the trend Ω4 > Ω2 > Ω6. Among all the glasses, TeWK glasses have highest values of J-O parameters which confirm that the bonding between RE ions and the surrounding ligands is highly covalent in nature. Under 429 nm excitation, the PL spectra show two sharp peaks at 488 nm and 670 nm in bluishgreen and bright red region corresponding to the transitions 3P0 → 3H4 and 1D2 → 3H5 respectively. Among these transitions, red color

glasses glasses glasses glasses glasses glasses

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emission predominates corresponding to the transition 1D2 → 3H5 (670 nm) for the TeWK glass. The radiative properties evaluated such as branching ratio, stimulated emission cross-section are also high for these two transitions in particular for TeWK glass. From the PL decay spectra, the experimental lifetimes have been measured and in turn used to evaluate the quantum efficiency of glasses. Among all the glasses, TeWK glass possesses highest values of quantum efficiency for the transition 1D2 → 3H5 at 670 nm. The emission cross-section, branching ratios and quantum efficiency measured for all these glasses allowed us to conclude that Tungsten tellurite glass with potassium fluoride as network modifier (TeWK glass) is aptly suitable to generate bright red laser at 670 nm.

[19] [20]

[21]

[22] [23]

[24] [25]

Acknowledgements [26]

This work is partly supported by High-impact Research initiative of IIT-Delhi, UK-India Education Research Initiative (UKIERI) and Department of Science and Technology (DST), Govt. of India, New Delhi. The authors, Prof. A. S. Rao (File Number: EMR/2016/007766), Dr. Sk. Mahamuda (File Number: ECR/2016/000376) and Dr. K. Swapna (File Number: ECR/2015/000335) are thankful to DST, Govt. of India, New Delhi for the award of a major research project to them.

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